1. Field of the Invention
The present invention relates to an electric variable resistor having a resistive element with a layered structure.
2. Description of the Prior Art
Japanese Laid-Open Patent No. 61 (1986) - 119622 shows a variable resistor in which the resistive element has a two-layer structure. The volume inherent resistance of the upper resistive layer is from 2 to 500 times the volume inherent resistance of the lower resistive layer. The variable resistor has good durability, resistive stability and output stability.
In conventional variable resistors, the resistive element is a two-layer laminate. The ends of the laminate are typically covered with a conductive paint or paste at the point where they are joined to an electrode or other electrical contact, making it difficult to independently measure the thickness of the individual layers. Once the resistor has been made, the only way to measure the thickness of a layer is to cut through the resistive laminate and view its cross section.
It is also extremely difficult to control the thickness of the individual layers of the resistive element during the manufacturing process. This is a serious problem from a quality assurance standpoint since, for example, if the resistive layer is too thin, the durability is reduced. If the resistive layer is thicker or thinner than the target value, the resistive value of the variable resistor becomes nonstandard.